SCR systems are employed to remove Nitrogen Oxides (NOx) from spent flue gas. Nitrogen Oxides pose pollutions problems, whereas the NOx can react in the atmosphere with water vapor to produce to acid rain, or react with sunlight to produce ozone. NOx forms during combustion of fuel to heat furnaces, including but not limited to, hydrogen reformers, vacuum heaters, platformers, and other process heaters. NOx can form in three ways during combustion: Thermal, Fuel, and Prompt NOx.
The purpose of a SCR system is to convert the Nitrogen Oxides into diatomic Nitrogen (N2). The system uses liquid ammonia and a porous catalyst to convert the NOx. The ammonia is injected with air into the flue gas at a controlled ratio, and the Nitrogen oxides react with the ammonia and oxygen to form diatomic Nitrogen and water. The catalyst increases the conversion rate to over 90%. The Nitrogen and water can then be safely released into the atmosphere.
The interior of a furnace is lined with refractory ceramic fiber (RCF), insulating refractory brick, or a combination of the two. The RCF is composed of amorphous silica, that when subjected to elevated temperatures, such as in a platformer, will devitrify into crystalline silica and other components. Due to the gas velocity in the furnace, the crystalline silica is picked up in the gas flow and carried downstream. Likewise, the gas velocity and water vapor can act as an abrasive to the insulating refractory, and by a process known as silica migration, particulates from the insulating refractory can also be gathered and carried. Also, during combustion of fuel, in particular, oil, particulates can be generated and also carried downstream in a process known as oil dusting, for example. These particles can be deposited on the surface of the catalyst and contaminate the generally porous surface of a catalyst, or on heat transfer vanes of a convection section of a furnace. These cannot only reduce the effectiveness of the catalyst, but can also damage or poison the catalyst, leading to reduced performance. In the case of heat transfer vanes, the particulates act as insulators; this contamination thus reduces the heat transfer efficiency of the unit.
In order to overcome such issues, various ways of cleaning the contaminated units have been employed. One known method involves use of a sonic horn, to generate sound waves to dislodge particulates from the contaminated unit, which are then expelled with the flue gas. However, these particles may also become lodged in the channels of the catalyst, or fall to the bottom of the unit and collect. Another method involves blasting, typically with dry ice, which as it sublimes in the region of the particulates causes them to be dislodged from the contaminated unit.